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<head>
  <meta charset="UTF-8">
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  <title>Document</title>
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<body>
  <script>
    // 1. 页面弹出输入框
    let r = prompt('请输入圆的半径：')
    // 2. 计算圆的面积（内部处理）
    let re = 3.14 * r * r
    // 3. 页面输出
    document.write(re)

    // NaN   not a number 
  </script>
</body>

</html>